1. Field of the Invention
The invention relates to a radar system and more particularly to a single radar system which is controllable in real time between two target location modes, the first mode being a two dimension mode where targets are located (detected and acquired) and tracked in azimuth and range and the second mode being a three dimension mode where selected tracked targets are located in a third dimension, elevation angle or height.
2. Description of the Prior Art
A wide variety of radar systems is presently available for locating target positions and for tracking these targets as they pass through a radar system's surveillance volume. For purposes of this discussion, the targets which are to be detected and tracked are capable of motion in three dimensions, for example, aircraft and missiles. These targets can be followed or tracked in two dimensions or three dimensions, however in the performance of some services, such as designating a target to a military fire control system or for vectoring a target between positions, three dimension target data may be required while other services require only two dimension target data.
There is a demand for a radar system which possesses both the capabilities of a two dimension (2D) radar and those of a three dimension (3D) radar. Two dimension surveillance, mechanically scanning radars are typically simple and economical and usually determine the azimuth and range of targets by generating a fan beam in the elevation plane and mechanically rotating that beam throughout a 360.degree. azimuth angle. This is the most efficient and economical Track While Scanning (TWS) system for detection and tracking of targets either manually or automatically. It has the shortest response time over a 360.degree. surveillance volume and has the least amount of equipment to provide the necessary signal-to-noise (S/N) ratio to ensure effective detection and tracking of targets. Most current day radars are of the 2D type. However, in many instances a target must be located in three dimensions, i.e., range, azimuth and elevation angle or height above sea level for the performance of a particular system mission. Single 3D radars produce the required 3D data, but they produce 3D data on all targets within the surveillance volume whether 3D data is required or not. They are typically complex, expensive, heavy in antenna weight, have low S/N ratios due to fewer target pulse repetition frequency (PRF) returns and provide the 3D target data by electronically scanning multiple pencil beams or by stacked pencil beams that cover the surveillance volume. As a consequence, radar energy and time are wasted due to the spotlighting of each 3D space resolution element in the surveillance volume since normally there are fewer targets in the surveillance volume for which 3D data is required than there are for which only 2D data is required. In addition, it requires considerably more time for the smaller pencil beamwidth 3D radars to scan the same volume as 2D radars therefore 3D radars have a slower data rate for detection and tracking of new targets in the surveillance volume. Generally, 3D radar designs are a compromise between detection range capability, data rate and resolution. Accordingly, the qualities of reliability, data rate, and detection probability are typically less than what would be available in a 2D radar system.
A variety of techniques is presently available to selectively obtain two dimension or three dimension data about targets in the radar system surveillance volume. One technique is the use of a search radar (2D radar) combined with voluntary target response. The search radar locates targets in azimuth and range and when a target is selected for determination of its third dimension, i.e. elevation, voice communication or other means such as IFF is used. In many cases however, this cooperative method is not feasible and it is desirable to locate targets in the third dimension without having to rely on a voluntary response from that target, especially where the target is uncooperative in nature or does not carry special reponse equipment.
Another technique known in the art is the combination of a two dimension search radar system with a height-finding radar system or a Fire Control System associated with a weapon system. These individual radar systems remain autonomous but cooperate in such a way as to selectively produce the 3D target data desired. In this technique, a search radar locates and tracks targets in two dimensions initially. When a target is selected for height data determination, a second radar system which has a narrow elevation beamwidth is given the azimuth and range of the target and this second radar locates the target additionally in elevation. Another application of this technique is also found in certain weapon fire control systems. In that application, surveillance efficient, automatic tracking, 2D target designation radars are used in cooperation with a 3D weapon fire control radar system. The 2D designation radar system will provide a range and azimuth of a selected target for designation to a fire control system when the target is within or near the weapon system capability envelope. The gathering of the elevation data, however, occurs after designation of the target to the weapon system 3D fire control radar. This causes a relatively slow response time to achieve an intercept on a designated target since the two dimension data of the target must be transmitted to the second radar and then the target must be located in elevation by the second radar before consideration of intercept point, weapon firing, and weapon guidance can occur. This is a procedure similar to the search and height finder radar operations discussed above. Obvious disadvantages are that two complete and independent radar systems are required with a suitable communication link between them and additional response time is required to provide the 3D data.
Another technique known in the art is the use of a 2D radar system having a height-finding attachment located on the same radar antenna pedestal. A radar system such as this is disclosed in U.S. Pat. No. 4,158,840 to Schwab. In Schwab, targets which are being tracked in two dimensions, i.e., azimuth and range, are designated to the height-finding attachment for determination of elevation position when required. Schwab discloses that the height-finding attachment consists of adding a second antenna to the radar by mounting it on the back surface of the existing search 2D antenna platform, and adding necessary autonomous height-finding radar transmitting/receiving and signal processing equipment to the two dimension search radar.
Adding an antenna to the search antenna platform has certain disadvantages, such as increased weight, wind resistance, alignment difficulties, additional rotary joint, etc. The addition of second transmitter/receiver equipment and a signal processor substantially increases the expense and size of the system. There are essentially two separate, independent radar systems operating on a common platform or pedestal with a communication link between them.
In summary, in providing 3D target data on selected targets for performing system missions such as civil or military aircraft control, designation/assignment of missile or aircraft targets to weapon control systems for area or point defense operations, etc., the prior art comprises the combination of two separate radar systems which operate over a mutual communication link or a single complex 3D radar system with single or multiple pencil beams. Typically, 3D data is not required on all targets within a surveillance volume and only those targets which are to be specially controlled or designated need to be identified by 3D data. Also, 3D data is typically not needed initially, but only when the selected mission targets are to be specially controlled or designated after they have been detected, acquired, tracked and identified for a particular time period. For a further discussion of prior methods, refer to M. I. Skolnik, INTRODUCTION TO RADAR SYSTEMS, 2d ed., 1980, pp. 541-547.
It is a purpose of the invention to provide a single radar system (i.e., one antenna, one receiver, one transmitter, one signal processor, etc.) which possesses both the capabilities of a two dimension surveillance/TWS radar system and those of a three dimension radar system and is manually (operator controlled) or automatically controllable between providing two dimension target data and three dimension target data either in the same scan or in subsequent scans depending on the mission need for elevation angle or height data.
It is also a purpose of the invention to provide a radar system possessing both the capabilities of a two dimension radar system and those of a three dimension radar system which is electrically and mechanically simpler and less expensive than prior art systems and techniques.
It is also a purpose of the invention to provide a method of adapting a conventional two dimension radar system which locates targets in azimuth and range to also provide elevation or height data of selected targets without impairing the normal functioning of the original 2D radar system.
It is a further purpose of the invention to decrease response time between selecting a target for determination of elevation data and obtaining that elevation data.
It is a further purpose of this invention to provide the 3D target data at a higher data rate than is achievable from a prior art mechanically rotating 3D radar or the prior art method of using two separate radar systems.
It is a further purpose of this invention to provide sufficient data to a weapon system such as a missile that does not require a constant external guidance from a continuous or sample data guidance system thereby eliminating the need for providing such a fire control system.